1. Field of the Invention
This invention relates to an underwater cable arrangement capable of being used, for example, in seismic testing.
2. Description of the Related Art
In marine seismic exploration, an underwater cable, commonly referred to as a streamer cable, is towed through the water by a vessel such as a surface ship. An array of hydrophones is mounted within the cable, and an acoustic source or gun is fired to force an impulse of compressed air into the water, creating a bubble. The collapse of the bubble generates acoustic pulses that radiate through the water and into the earth. Reflections of the pulses off geologic structures are picked up by the hydrophones, and data representing the detected reflections are transmitted to the towing vessel. By analyzing the reflections, it is possible to discover subsea formations of oil or gas.
Various devices requiring electric power for their operation are mounted on the exterior of the underwater cable. For example, in order to accurately control the depth of the cable as it is being towed through the water, depth control mechanisms, commonly referred to as xe2x80x9ccable-leveling birdsxe2x80x9d, are attached to the cable at intervals along its length. The depth control mechanisms are equipped with adjustable diving planes, the angles of attack of which can be varied by motors in the depth control mechanisms so as to maintain the cable at a desired depth. Another type of external device frequently mounted on an underwater cable is an acoustic ranging device, which together with other acoustic ranging devices is used to determine the locations with respect to the towing vessel of various points along the underwater cable to permit the exact shape of the cable during towing to be determined. Yet another type of commonly used external device is a magnetic heading sensor (also referred to as a magnetic compass), which determines the heading of the underwater cable at the point where the heading sensor is attached to the underwater cable.
It is possible to connect such external devices with a source of power aboard the towing vessel by means of wires passing through the skin of the cable, but the necessity of forming holes in the skin for the wires can lead to leakage of water into the interior of the cable, which is highly undesirable. Therefore, the external devices are usually self-powered by their own internal batteries. However, batteries have a number of serious drawbacks. First, the batteries have a limited life span and must be replaced every one to three months. Not only is the replacement of the batteries time-consuming, there is also the risk of exposing electric circuitry within the external device to sea water during replacement. In addition, lithium primary batteries, which are used because of their longer operating life compared to other batteries, are expensive, and spent batteries must be retained for proper disposal and not randomly discarded. Furthermore, each time the cable is reeled in to replace batteries in the external devices, it is subjected to stresses which frequently result in damage to the cable. Therefore, it is preferable to reel in the cable as infrequently as possible.
Another shortcoming of conventional external devices used with underwater cables is that the entire device must be removed from the cable before the cable is rolled up onto a reel on the deck of a towing vessel in order to prevent damage to the external devices and the cable. When the cable is to be redeployed, the external devices must be reattached to it. In rough seas, it can be extremely difficult and often dangerous for workers to remove the external devices from or reattach them to the cable on the deck of the towing vessel. Furthermore, onboard storage space, often a precious commodity, must be provided for the external devices on board the towing vessel. In addition, the reeling in or paying out of the underwater cable must be stopped during removal or reattachment of the external devices, so deployment and retrieval of the cable are time consuming.
In accordance with one aspect of the present invention, devices for controlling the position of an underwater cable comprise a body, first and second actuators, and a pair of wings. The body is stationarily mounted to the underwater cable and the first and second actuators are disposed in the body. Each wing has an axis of rotation, and the wings are coupled to the first and second actuators to control the depth and the horizontal position of the underwater cable in the water.
In accordance with another aspect of the present invention, devices for controlling the position of an underwater cable comprise a body, a pair of wings, sensors, and a pair of actuators. The body is connectable to the underwater cable and the pair of wings are attached to the body. The pair of wings include a first wing having a first axis of rotation and a second wing having a second axis of rotation. The first wing is rotatable about the first axis of rotation and the second wing is rotatable about the second axis of rotation. The sensors produce signals from which angular positions of wings can be determined. The pair of actuators are responsive to the sensors and co-act with the wings to adjust angular positions of the wings to control the depth and the lateral position of the underwater cable in the water.
In many embodiments the first and second actuators are interconnected to the pair of wings to control the angle of attack and the roll angle of the wings. Thus, embodiments of the invention are capable of controlling the depth of the underwater cable beneath the water surface and may be used to steer the cable by controlling the horizontal or lateral position of the cable within the water.
In many embodiments, the exterior devices mounted on an underwater cable are capable of being powered primarily or entirely by inductive coupling between the coil within the underwater cable and a coil in the external device. Therefore it is not necessary to have any wires connecting the external device with the interior of the underwater cable, and the external devices do not require their own internal battery as a primary source of power. Batteries can be omitted from the external device, or can be used merely has a backup in case the supply of power from the coil within the cable is interrupted. Therefore, the load on any batteries in the external device is small, and the batteries can be used for long periods within requiring replacement. Therefore, the underwater cable can be operated for long periods without having to be retrieved on board a towing vessel, resulting in increased operating efficiency and reduced wear and tear on the cable.
In many embodiments, all or a portion of a external device can be left attached to the underwater cable when the cable is wound around a wheel or drum for storage. The ability to leave the external devices attached to the cable greatly reduces the amount of labor and the time required for deploying or retrieving the cable, resulting in greatly increased operating efficiency. The external devices may also be structured such that all or a portion of the devices can be quickly and easily mounted on and detached from the underwater cable when desired without the need for any tools.
In many embodiments, a coil support device is cable of supporting a coil within an underwater cable such that substantially no bending stresses are applied to a core of the coil during operation of the underwater cable, even when the cable is bent, such as when it passes over a roller on a deck of a towing vessel, or is subject to impacts. Therefore, even a coil having a fragile ferrite core can be protected from damage.
The external devices of an underwater cable arrangement can perform a wide variety of functions, including but not limited to, sensing the heading of the cable, performing acoustic ranging, and controlling the depth or the position of the cable in the water.